Trane XL20i Defrost Cycles

We have had a problem with the Trane XL20i going into way too many defrost cycles. Trane has been out with their tech guys and checked everything but it still will defrost way too many times. The last time we had 40 degree we ather, the four ton ran then defrosted, ran, then defrosted three times. I f you have run into this problem and solved it, please let me know. It has been going on for years. Trane actually replaced the whole outside unit thi s January but we are still having the problem.
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